MEDIANAS EMPRESAS EN LA PROVINCIA DE PICHINCHA

A O c _{5 1 .4} s O E u 4 8 .4 I 5 5 /1 8 BOXRH 220 days \ 5 5 /1 8 OXRH 220 days Tem perature (°C)

end groups. Water could be attracted and interacted with the PEG esters part in the bases. A few hypotheses have been proposed concerning the interaction of PEGs chain and water. These included, trihydrate and monohydrate complex formation through the ether group of the PEG chains (Graham, 1992; Graham et al, 1989) or else the formation of a simple binary crystalline eutectic of PEGs and water (Hager and Macrury, 1980). These proposed mechanisms involve only the PEG chains and not the chain ends hence can be applied to the interaction of gelucires and water, especially in gelucire 55/18 which shares many of PEGs properties. However this is, at best, a speculation.

A more simple mechanism involving only at a macroscopic level, however, will be proposed here. The DSC curve of 50/13 sample kept at 80% RH (Figure 3.29 Bottom) was very similar to that of A50/13 (Figure 3.25 and 3.27). Both samples had two distinctive peaks at 37.1°C and 43.5°C, which represented 2 relatively stable fractions (since both aged and freshly prepared A50/13 had these 2 peaks). This similarity could not be purely coincidental.

It has already been shown that slow cooling allowed fractional crystallisation, ie. the crystallisation of the components with similar properties (or segregation). Water could act as a medium to promote this segregation by allowing components with similar polarity to break from the existing phases, e.g. solid solutions, to form new phases or even to segregate into single component. In other words, the presence of water provided greater mobility of the molecule especially PEG esters (because of its polarity) to find their compatible phases. It should be noted that the role of water to PEG esters in 50/13 was similar to that of liquid fats to glycerides proposed for 43/01 in 3.3.3. This explanation can be extended to gelucire 50/02.

Chapter 3 ...D S C Studies /171

4. Conclusion

Polymorphism of glycerides and the phase behaviour of gelucires have important roles in their thermal behaviour. These two phenomenon cannot be separated and have to be considered together in order to understand the thermal behaviour of the bases.

The cooling rate used to solidify gelucire bases dictated their microstructures and hence their thermal properties. Slow cooling promoted segregation of components with very similar properties, e.g. chain length, melting point, polarity, etc., allowing them to crystallised together. This segregation or fractional crystallisation, which was also found in solvent crystallisation, resulted in various fractions of fat melting independently. On the other hand, rapid crystallisation of the melt tended to promote the formation of mixed

crystals resulting in a more homogeneous fat. Slow cooling also allowed the

crystallisation of higher polymorphic forms of glycerides while metastable forms crystallised with rapid cooling.

The difference between the microstructure of slowly cooled and fast cooled samples could be detected by the shape and the peak in DSC curves and AHf values. Since the slowly cooled and the fast cooled samples had different solid fat contents at each temperature, the use of SFC graphs proved to be very helpful in the understanding of the system.

The cooling rate which was considered to be slow for one fat, e.g.54/02, may not be so for other, e.g. 43/01. Therefore, the effect of segregation may be obvious in one

fat (54/02) but not the other (43/01). This depended on the composition of fats

concerned (e.g. the fatty acid chains, the presence of PEG esters), which determined their crystallisation. However, if crystallisation was prolonged, segregation would certainly occur. The same could be said for the fast cooling rate, e.g. gelucire 50/13 had a tendency to segregate even the crystallisation was very fast. The effects of cooling rate on the thermal properties of gelucires are similar to those reported for milk fat (Mulder,

1953; deMan, 1961a, b; Timms, 1980) and confectionary fats such as cocoa butter (Lovegren et al, 1976; Aronhime and Garti, 1988).

On ageing of gelucires, polymorphic transformation occurred, resulting in the phase changes within the bases, e.g. the breaking of the existing and the formation of the new solid dispersions. In gelucires which consist solely of glycerides, these changes would finally lead to bases with the same thermal properties regardless of cooling rates used. On the contrary, even at 280 days some slowly cooled and fast cooled gelucires (50/13, and to a lesser extent in 50/02) still had different thermal properties. The presence of components with very different properties (glycerides and PEG esters) was responsible. Tempering was also found to accelerate the process of ageing or to promote segregation depending on the temperature used.

It was also postulated that the presence of certain components such as diglycerides or monoglycerides may inhibit the polymorphic transformation into the most stable form. Moreover, the presence of water in the system could lead to the bases with different thermal properties. Water exerted its effect only on gelucires which contain PEG esters.

Solvent crystallisation resulted in fractional crystallisation of fats. In addition its usually but not always (as seen in 43/01) lead to the crystallisation of stable polymorphic forms of glycerides hence samples with high AHf values.

No attempt was made in this study to relate the polymorphic form found with a , B' or B polymorph of pure triglycerides. Such correlation is impossible without data from other techniques, e.g. X-ray powder diffraction and besides may not be important for the understanding of the systems. It is also questionable that X-ray technique will be helpful at all. In such complex mixtures of glycerides, there is likely to be a , B', B polymorphs which belong to different components crystallised together at all times.

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Flow & Mechanical